skip manifest validation for default kubectl deployer

[gsquared94]

Fixes: #6254

Description

In #6043 we introduced validation for the kubectl deployer manifests. This breaks backward compatibility for users who were using skaffold run without any deployer defined. Although that doesn't seem like a valid usecase for the run command, for versions <= v1.26.0, skaffold run command worked the same as skaffold build command in the absense of a deployer stanza in the skaffold config.

The default deployer (./k8s/*) gets added when no explicit deployer is defined, which now fails the validation introduced in #6043.

To keep backward compat behavior, this PR skips the manifest validations when only the default kubectl deployer is defined.

[codecov]

Codecov Report

Merging #6294 (7a1a8a6) into main (533016e) will decrease coverage by 0.06%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##             main    #6294      +/-   ##
==========================================
- Coverage   70.86%   70.79%   -0.07%     
==========================================
  Files         490      494       +4     
  Lines       22162    22334     +172     
==========================================
+ Hits        15704    15811     +107     
- Misses       5441     5503      +62     
- Partials     1017     1020       +3     

Impacted Files	Coverage Δ
pkg/skaffold/schema/validation/validation.go	86.20% <25.00%> (-0.70%)	⬇️
pkg/skaffold/docker/image.go	66.55% <0.00%> (-2.33%)	⬇️
pkg/skaffold/docker/parse.go	87.39% <0.00%> (-0.85%)	⬇️
pkg/skaffold/deploy/docker/deploy.go	0.00% <0.00%> (ø)
pkg/skaffold/docker/logger/log.go	23.07% <0.00%> (ø)
pkg/skaffold/docker/tracker/tracker.go	85.29% <0.00%> (ø)
pkg/skaffold/event/v2/debugging.go	100.00% <0.00%> (ø)
pkg/skaffold/docker/logger/formatter.go	75.00% <0.00%> (ø)
...skaffold/kubernetes/debugging/container_manager.go	60.31% <0.00%> (+1.30%)	⬆️
pkg/skaffold/event/v2/event.go	83.79% <0.00%> (+3.79%)	⬆️
... and 1 more

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[aaron-prindle]

LGTM